Recent progress in battery development have required the use of high surface area electrodes for large energy and power density capabilities. In such high energy density cells, finely divided powders of cathode component materials, such as lithium metal oxides (LiM.sub.x O.sub.y), are deposited over a large surface area and are impregnated with an electrolyte.
For maximum surface area and optimal arrangement of the cathode component materials on a suitable substrate, it is desired that the electrode component materials be finely divided and of a relatively narrow particle size distribution. Porous particles are particularly desirable because they provide a large surface area to volume ratio. Agglomeration of individual particles is not considered desirable because surface area is minimized and irregularly shaped agglomerates do not stack well, which is in conflict with the need for small physical size in the electrode. Similarly, wide distribution of particles sizes have a significant deleterious effect on energy density of the cell.
Finely divided powders have been difficult to obtain and to deposit with control of layer thickness and uniformity. For preparation of powders of cathode component materials, such as LiM.sub.x O.sub.y, conventional thermal processing followed by grinding and classification are commonly used. In such processes, metal oxides and lithium and/or lithium salts, for example, are ground together to commingle and comminute the materials, which are then reacted at high temperatures to promote formation of the LiMn.sub.y O.sub.x crystalline structure. Repeated steps of grinding and thermal processing are often required to obtain significant conversion of the starting materials to the desired lithium metal oxide. However, even after prolonged heat treatment and grinding, conversion may be incomplete and the particle size of the product may be undesirable. The prior art has heretofore lacked a method for the production of fine particle cathode materials for the fabrication of electrode structures.
Thin electrode structures have been prepared by applying a fine particle slurry onto a substrate, in particular by ultrasonic assisted spraying, and by spraying dry particles onto a substrate (see, U.S. Pat. No. 5,158,843 to Batson et al. and JP 4104461 A, for example). In such methods, however, particle composition, particle size, and particles size distribution are determined during earlier manufacture of the particles, which is subject to the limitations described above.
Transition metal oxide layers have been applied to substrates using pyrolytic decomposition, in which a precursor solution is sprayed onto a heated substrate where it decomposes to form a metal oxide coating (see, U.S. Pat. No. 4,960,324 to Brown). However, pyrolytic decomposition does not permit the formation of finely divided powders having a large surface area.
It is desirable to provide a method for the preparation of electrode materials of small dimension and large surface area which overcome many limitations of the prior art as described herein above. It is further desirable to provide fine particle electrode materials of small dimension and possessing improved electrical properties.